J Integr Plant Biol. ›› 1964, Vol. 12 ›› Issue (3): -.

• Research Articles •    

Embryogeny of Metasequoia

F. H. Wang and N. F. Chien   

Abstract: The material of Metasequoia glyptostroboides Hu et Cheng for the embryological investigation was collected in 1960 in its native land, Shui-sha-pa, Hupei province of central China. The development of the female gametophyte is described with reference to the extent of the free nuclear stage, followed by an alveolar growth. The free nuclei actually counted in two ovules were 1,764 and 1,872, and those at one cell layered stage were estimated about 4,000. The megaspore wall is one-layered and about 1.6 μ in thickness. The archegonial initials have their origin in the first walled cells and they can be distinguished from other gametophytic cells by their larger size. The archegonia are usually grouped into an archegonial complex which is terminal in position. The number of the archegonia varies from 8 to 11, and some abortive archegonial initials may be included among them. Four out of about 400 ovules examined possess a lateral archegonial complex or a single lateral archegonium in addition to the terminal one. The division of the central cell gives rise to the egg and the ventral canal nucleus which disintegrates usually soon after its formation. Occasionally the ventral canal nucleus may be retained in the upper part of the archegonium. It is probable that the presence of some nuclei in the upper part of some archegonia throughout the whole proembryonal stage is attributed to the further division of the persisting ventral canal nucleus. The male gametophytes were observed with the usual trio in the pollen tube–the spermatogenous cell (the body cell), the sterile nucleus (the stalk nucleus) and the tube nucleus. At first the trio appears to be of the same size, then the spermatogenous cell grows larger and larger on its way through the nucellus and finally reaches its maximum size before its division. The division of the spermatogenous cell takes place after it reaches the megaspore membrane and just before fertilization. Two sperms are identical. Fertilization took place between June 22–30, 1960. Only the nucleus of the sperm actually enters into the archegonium. It is much smaller than the egg nucleus. When it approaches the egg, the latter forms a concave surface facing the direction of the sperm nucleus. The proembryo begins its development with three free nuclear divisions; walls are formed at eight nuclei stage. The eight cells of the proembryo are arranged in the single open tier and the primary embryo cells, the relative numbers of which are usually 4:4 or 6:2, very rarely 5:3. The cells in the open tier are not walled above, while the primary embryo cells are arranged roughly in 2 tiers. With the simultaneous divisions of the cells in the open tier, two regular tiers are formed. The cells in the upper tier are still open to the cytoplasm above, while the tier below forms the prosuspensor. The primary embryo cells may divide once more and the divisions are not necessarily simultaneous. Some of them may remain undivided, thus the total number of the embryo cells varies from 3 to 12, with 4–7 as the usual range. The elongation of the prosuspensor pushes its terminal group of embryo cells into the female gametophytic tissue. The embryo cells cut off primary suspensor cells which elongate backwardly. As the prosuspensor and the primary suspensor cells continue to grow and become folded due to rapid elongation, the terminal cells at the primary suspensor ends divide in various ways and develope into multicellular embryos. The apical cell activity is generally not conspicuous. The embryonal tubes which elongate backwardly from the upper part of the embryos are usually formed when the embryos reach 30–40 cells stage. Just like the case in other conifers, in Metasequoia more than one archegonium may be fertilized at the same time, thus the simple polyembryony prevails. And in the same embryo system developed from the same fertilized egg, 3–12 individual embryos may be developed, thus the cleavage polyembryony is the usual case. Embryos are, therefore, rather many in early embryogeny. However, only a few of them may survive through embryonic selection and only one embryo is usually found in the mature seed as all the other embryos become collapsed during the course of embryogeny. The late embryogeny of Metasequoia is fundamentally similar to that of other conifers. Metasequoia agrees with Glyptostrobus in the well developed hypocotyl, in the lack of a pith and in the weak development of the root cap-suspensor region, but it differs from the latter in the number of the cotyledons. The embryogeny of Metasequoia resembles, in general, that of Sequoiadendron more nearly than that of any other conifer thus far investigated. In Sequoiadendron the cells in the upper tier (rosette tier) of the proembryo are walled above and may divided to produce “rosette embryos”. The archegonial complex is lateral in position and the number of archegonia is rather numerous, while the archegonial complex of Metasequoia is terminal in position and the number of archegonia is fewer. The appearance of the embryonal tubes is later in Metasequoia. Aside from these important differences, the similarities in embryogeny between Metasequoia and Sequoiadendron are very striking. Both have a similarity in the development and the general structure of the proembryo; both form a prosuspensor; both have a similarity in the formation of primary suspensors and the constant occurrence of cleavage polyembryony. The embryogeny of Metasequoia also resembles that of Sciadopitys in certain respects. Both form a prosuspensor and primary suspensors; both have a similarity in the formation of cleavage polyembryony. They differ, however, from each other in such important features as the way of grouping of the archegonia, the formation of the proembryo, the formation of “rosette embryos”, the extent of cleavage polyembryony and so on. The present work, therefore, does not support the conclusion of Schwarz and Weide (1962) that Metasequoia, Sequoia and Sequoiadendron should be reduced to one genus-Sequoia Endl. and it also does not support the proposal of Hu et Cheng (1948) that a new family Metasequoiaceae should be adopted. It appears that the genus Metasequoia should remain under Taxodiaceae.

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